In recent years, the demand for high-strength steel (high-tensile steel) capable of contributing to weight reduction of structures and the like has been growing in the field of automobiles, home appliances, and building materials. The technology of high-tensile steel may possibly be capable of manufacturing a high-strength steel strip with good stretch flangeability by adding Si to steel and suggests the possibility that a steel strip with good ductility can be provided because retained γ is likely to be formed when Si or Al is contained.
However, when a high-strength cold-rolled steel strip contains easily oxidizable elements such as Si and Mn, there are problems such as poor surface appearance and failure in chemical conversion treatability such as phosphate treatment because the easily oxidizable elements are enriched on surfaces of the steel strip during annealing and therefore oxides of Si, Mn, and the like are formed.
For galvanized steel strips, when a steel strip contains easily oxidizable elements such as Si and Mn, there is a problem in that the wettability is impaired and plating defects are caused because the easily oxidizable elements are enriched on surfaces of the steel strip during annealing and therefore oxides of Si, Mn, and the like are formed. Furthermore, there is a problem in that the alloying rate is reduced during alloying subsequent to plating. In particular, Si significantly reduces the wettability between the steel strip and a plating metal if oxide films of SiO2 are formed on surfaces of the steel strip. Furthermore, the SiO2 oxide films act as barriers to the diffusion of a base metal and a plating metal during alloying. Therefore, Si is likely to particularly cause problems such as impairments in wettability and alloying treatability.
A potential way to avoid the problem is to control the oxygen potential in an annealing atmosphere.
For example, Patent Literature 1 discloses a method for increasing the oxygen potential in such a way that the dew point of a region from the rear end of a heating zone to an soaking zone is controlled to a high dew point of −30° C. or higher. This method has the advantage that the method produces some effect and it is industrially easy to adjust the dew point to the high dew point. However, the method has a disadvantage that it is not easy to manufacture the type of steel which is unsuitable for operation at a high dew points (for example, Ti-IF steel). This is because it takes a very long time to change the annealing atmosphere from a high dew point to a low dew point. Furthermore, the method produces an oxidizing furnace atmosphere, and incorrect operation results in pick-up defects due to the deposition of an oxides on rollers in the furnace or damage to furnace wall.
Another potential way is to control the oxygen potential to be low. However, since Si, Mn, and the like are very oxidizable, it is very difficult to stably create an atmosphere having a low dew point of −40° C. or lower which is excellent in suppressing the oxidation of Si, Mn, and the like in such a large-size continuous annealing furnace in a CGL (continuous galvanizing line) or a CAL (continuous annealing line).
For example, Patent Literatures 2 and 3 disclose techniques for efficiently achieving an annealing atmosphere with a low dew point. These techniques are applied to relatively small-size furnaces such as one-path vertical furnaces and do not take into account that a steel strip containing an easily oxidizable element such as Si or Mn is annealed in a multi-path vertical annealing furnace such as a CGL or a CAL.